Exhaust gas generated by combustion of fossil fuels in furnaces, ovens, and engines contain, for example, nitrogen oxides (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO), which are undesirable pollutants. Vehicles, e.g., diesel vehicles, utilize various pollution-control after treatment devices (such as a NOx absorber(s) and/or Selective Catalytic Reduction (SCR) catalyst(s)), to reduce NOx. For diesel vehicles using SCR catalysts, NOx reduction can be accomplished by using ammonia gas (NH3). In order for SCR catalyst to work efficiently and to avoid pollution breakthrough, an effective feedback control loop is needed. To develop such technology, the control system needs reliable commercial NOx sensors.
Some NOx sensor designs use an impedance oxide method to measure the partial pressure of NOx. In this design, NOx gases interact with semiconducting oxides, thereby causing an electrical impedance change in the oxides. These sensors cannot effectively maintain calibration. Further, these sensors experience selectivity problems wherein other gasses; for example, CO, hydrogen (H2), HC, NH3, CO2, and water (H2O), will interfere with NOx sensing.
Therefore a cost effective sensor that can reliably measure NOx under exhaust gas conditions would benefit the control system.